1. Field of the Invention
The present invention relates to the production of liquid fuels, and more particularly to a hybrid plant for low cost production of liquid fuels from hydrogen and carbon monoxide containing streams using light fossil fuel (for e.g. methane, natural gas, liquefied petroleum gas, naphtha), and solid feedstock such as biomass, coal, petroleum coke and the like.
2. Description of Related Art
Liquid fuels can be produced from solid feedstock materials by first gasifying the solid feedstock material to form hydrogen and carbon monoxide containing stream (gasifier-syngas) that is further treated to form desired liquid fuel product. Typically a single train gasification facility employing known gasifier technologies such as entrained flow, fixed or moving bed and fluidized bed will have an annual availability of less than 90%.
Higher gasifier availabilities are desired to increase product income to offset the capital cost of the overall project. A second and even a third gasification unit (two operating and one spare) are included in such projects to improve overall product availability and project economics. The inclusion of additional solids processing and gasification trains is capital intensive. In addition, conventional gasification systems for liquids production are generally designed with startup/auxiliary boilers to provide steam required for initiating unit operations such as feedstock drying, gasification where steam is often used as a moderator to control gasifier temperature, and regeneration of solvent used in acid gas removal system. These boilers add to the cost of the overall project.
At design conditions the gasifier-syngas produced from gasification of solid feedstock material such as coal, biomass, petroleum coke and the like typically has a molar ratio of hydrogen to carbon monoxide (H2:CO) considerably less than two, and often less than one. The H2:CO ratio considerably less than two is not optimal for converting gasifier-syngas into liquid fuel product such as Fischer-Tropsch liquids, methanol and the like. To improve liquid fuel product yield several options are available to adjust the H2:CO ratio in the feed to the liquid fuel production unit. For example, one approach is to subject the gasifier-syngas to water gas shift reactions to first increase its H2:CO ratio then convert into liquid fuel product; another approach is to mix with a hydrogen and carbon monoxide containing gas that has a higher H2:CO ratio to form a mixture containing desirable H2:CO ratio; yet another approach is to add hydrogen to increase H2:CO ratio, yet another approach is some combination of these.
Liquid fuels can also be produced from light fossil fuels by steam methane reforming (SMR), autothermal reforming (ATR), combinations of steam methane reforming and autothermal reforming (also referred to as secondary reforming) to form a hydrogen and carbon monoxide containing stream (synthesis gas) that is further treated to form desired liquid fuel product. Typically, such a light fossil fuel conversion unit containing SMR, ATR or combination of SMR and ATR will have an annual availability of greater than 98%.
U.S. Pat. No. 4,407,973 to van Dijk et al. discloses an integrated system containing two methanol plants. The first methanol plant is designed to use a hydrogen and carbon monoxide containing gas produced by a steam methane reformer. The second methanol plant is designed to use two gases, one of which is the purge gas from the first methanol plant and the other containing hydrogen and carbon monoxide produced by high temperature partial oxidation of heavy carbonaceous fuel such as coal. The two plants are designed such that the methanol output of the integrated system when both plants are operational lies between 1.45 and 1.75 of the design capacity of the first methanol plant that employs steam methane reformer. The hydrogen and carbon monoxide containing gas from steam methane reformer fed to the first methanol plant contains hydrogen in excess of that required for methanol synthesis reactions. The excess hydrogen ends up in the purge stream, and has considerably higher hydrogen to carbon monoxide ratio than in the feed. The second methanol plant in effect is sized to utilize the purge stream in combination with hydrogen and carbon monoxide containing gas produced by high temperature partial oxidation of coal to output additional methanol between 45 and 75% of the design capacity of the first methanol plant. When the high temperature partial oxidation unit is unavailable then the integrated system output is about 57% (1/1.75) or about 69% (1/1.45) of the integrated system design capacity.
U.S. Pat. No. 4,443,560 to Le Blanc et al. discloses an integrated system containing two methanol plants. The first methanol plant is designed to use a hydrogen and carbon monoxide containing gas produced by a steam methane reformer. The purge gas from the first methanol plant is first treated in a secondary reformer by contacting with steam and oxygen in the presence of reforming catalyst and then passed on to the second methanol plant. The second methanol plant uses this treated (reformed) purge gas as well as a hydrogen and carbon monoxide containing gas produced by high temperature partial oxidation of heavy carbonaceous fuel such as coal. It also discloses that if the first methanol plant is not present or a first methanol plant is not desired then the hydrogen and carbon monoxide containing gas leaving the steam methane reformer is passed directly to a secondary reformer and treated with oxygen and steam in the presence of reforming catalyst. The effluent from the secondary reformer is cooled and subjected to condensate removal, then mixed with hydrogen and carbon monoxide containing gas produced by high temperature partial oxidation of heavy carbonaceous material such as coal. The resulting mixture is then fed to the second or the only methanol plant.
U.S. Pat. No. 7,863,341 B2 to Routier describes an integrated process for producing hydrogen and carbon monoxide containing gases from two sources to form a gas containing optimum hydrogen to carbon monoxide ratio for use as feed in the Fischer-Tropsch process. The first source such as coal, peat, bitumen has low hydrogen to carbon ratio. This first source is contacted with oxygen in a gasifier to produce syngas that has a ratio of hydrogen to carbon monoxide typically between 0.5:1 and 0.8:1. The second source such as natural gas, coal bed methane gas has high hydrogen to carbon ratio. The second source is reacted with steam in a steam methane reformer (SMR) to produce syngas that has a ratio of hydrogen to carbon monoxide typically between 5:1 and 6:1. The syngas from the gasification unit and the SMR are combined in appropriate proportions to form Fischer-Tropsch process feed gas that has a ratio of hydrogen to carbon monoxide of about two.
U.S. Publication 2011/0218254 A1 to Chakravarti et al. discloses process configurations to produce feed gas for producing Fischer-Tropsch liquids, ethanol, etc. The feed gas is formed by mixing appropriate proportions of hydrogen and carbon monoxide containing streams from gasification of solid feedstock and reforming of light fossil fuels and hydrocarbon containing byproduct streams from the liquid fuel production unit. In some configurations the effluent from the gasification of solid feedstock is subjected to partial oxidation to convert heavier hydrocarbons into at least hydrogen and carbon monoxide.
The need continues to exist for a liquid fuel production method and plant that increases product income to offset the capital cost of the overall project. It is an object of the present invention to produce liquid fuel product from hydrogen and carbon monoxide containing gas formed by converting solid feedstock using oxygen and by converting light fossil fuels using oxygen to provide improved product availability and project economics.
It is also an object of the invention to maintain liquid fuel production at near nameplate capacity under a variety of conditions, including when solid feedstock conversion unit is not operational or operating at below the nameplate capacity. Equipment utilization and productivity is maximized by maintaining the liquid fuel production within acceptable ranges of design capacity. Project cost is reduced by achieving high availability without extra equipment such as spare gasifiers.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.